Recently, many mobile appliances have been equipped with RF communication capabilities, which provide increased functionalities and opportunities for novel applications. A key problem, however, is the need to often change or charge batteries due to the high energy requirements of RF communication. Short battery life is a major obstacle that precludes the technical viability needed to create new innovative products.
Recent advances in signal processing allow flexible tradeoffs in the design of RF communication circuits, such as throughput, sensitivity, signal-to-noise ratio (SNR), and bit-error rate (BER).
The Shannon-Hartley theorem predicts, for a given information channel capacity, a tradeoff between bandwidth and transmitted power (Wikipedia: Shannon-Hartley Theorem); generally, broader bandwidth use allows lower transmission power, or a higher data rate requires higher transmission power. Normally, a tradeoff between bandwidth efficiency and transmitted power is realized by processing gain using high-speed digital signal logic circuits. While this approach improves efficiency during active communication, the overall power consumption remains high due to the receiver's long operating period. During operation, significant power is expended to listen for asynchronous transmissions. This power and energy load is most pronounced in small mobile devices and is very difficult to reduce due to competing laws of physics. Polling style communication, for example, has been developed to reduce the duty cycle of the continuous link between participating radios. Other approaches, such as active communication and burst-synchronized polling at regular intervals, may be used to reduce average power. (See, e.g., FIG. 1, which shows typical average power consumption as a function of percentage duty cycle in transmitters and heterodyne or non-heterodyne receivers).
In the detailed description below, the term “latency” refers to the delay between the time an information source is ready to send information and the time the information is actually received by a receiver. The term “asynchronous communication” refers to a protocol by which a transmitter device initiates communication with a receiver at any time, without regard to any constraint of local time, or a clock signal, or the state of an intended receiver device. That is, in asynchronous communication the transmitter device initiates transmission at will, independent of the timing at the receiver.
For an appliance that communicates infrequently but demands low latency asynchronous communication, reducing average power consumption by reducing active radio duty-cycle is constrained by the competing latency requirements. This phenomenon is one example of the conflict between functional requirements and battery life.
In the context of the present disclosure, the term ‘appliance’ refers to an apparatus or a device that performs a specific function or set of functions, and that further includes within its structure a radio communication apparatus, a radio communication system or a radio apparatus, as further described herein. One example of an appliance in this context is a wireless garage door opener.
Heterodyne detection is a method for detecting an electromagnetic signal using a reference frequency. The heterodyne method uses a non-linear frequency mixing effect to beat a locally generated reference signal with the received signal, in the process translating the received signal carrier frequency to a different frequency. The reference signal source is also known in the art as the “local oscillator” (LO). The nonlinear device that combines the received signal and the local oscillator is known as the ‘mixer’ in the art of radio communication. Heterodyne-based receivers exhibit excellent selectivity and sensitivity.
Reduction of power consumption in a heterodyne receiver is limited by the power that is required to operate the high-frequency LO that, in turn, drives the mixer.